Gauges having the above-mentioned features are known in the art. An example is shown in International patent application published with n. WO-A-9712724 that refers to a specific apparatus, for checking the diameter of crankpins during their orbital motion about a geometric axis. The gauge—that is part of the apparatus and is shown in FIGS. 6 and 7 of the international application—includes a V-shaped reference device that rests on the pin to be checked and a tubular guide casing. A transmission rod axially translates in the guide casing and carries a feeler that contacts the surface of the crankpin to be checked. The displacements of the rod are detected by a measuring device with an inductive transducer that includes a first part integral with the tubular casing and a second part integral to and movable with the transmission rod.
In the specific application that is shown in the above-mentioned international patent application, for checking a crankshaft being worked on a grinding machine, the fixed part of the gauge—including the tubular casing, the V-shaped reference device and a part of the inductive transducer—is coupled to a support system allowing the whole gauge to perform the displacements that are needed to keep the contact with the pin to be checked while the latter orbitally moves.
The tubular casing has an opening through which the end of the transmission rod carrying the feeler projects. A sealing device closes such opening in order to prevent coolant and other foreign matter that is present in the working environment from getting into the casing. More specifically, the sealing device is made up of a metal bellows having its ends fixed to the rod and the casing, respectively, that has also the function of preventing axial rotations between rod and casing, so preventing the feeler from undertaking improper angular positions.
As an alternative, known gauges include tubular external gaskets of different shape and/or material, e.g. made of rubber, arranged between the casing and the movable element carrying the feeler. These gaskets perform a sealing action but have no substantial role as far as anti-rotation is concerned.
The increasing demand for internal combustion engines featuring the utmost compactness leads to the production of components having smaller and smaller dimensions. One of such components is the crankshaft, that requires extremely accurate dimensional checkings.
It can be hard or impossible to use the apparatus shown in the above cited patent publication WO-A-9712724 for checking crankshafts featuring very small nominal dimensions. For instance, in case that the crankpin to be checked has extremely small length, it is necessary that consequently small be the thickness of the part of the gauge including the V-shaped reference device, the feeler, the corresponding ends of the rod and of the casing, and the sealing system. In fact such part of the gauge, during the checking operations, must be in touch with or very close to the surface of the pin, i.e. has to be arranged between the walls of the mutually facing cheeks at the ends of the pin. It is especially troublesome to reduce the dimensions of the sealing system when the available allowance is less e.g. than 8-10 mm. In fact, it is problematic to get metal bellows or different tubular gaskets featuring suitable transversal size, safe connection to both the mutually movable parts and right properties ensuring the needed characteristics as regards compliance and tightness.